Mass data storage devices include well-known hard disk drives that have one or more spinning disks or platters carrying a magnetic media onto which data is recorded for storage and subsequent retrieval. One or more write heads are provided that fly over the magnetic media to generate magnetic fields in response to write currents applied to the heads to change the magnetic domains of the magnetic media corresponding to the data being written. However, the write heads themselves contain materials that may become magnetized as the write currents are applied to them. This residual magnetism may degrade the performance of the read and write operations to be carried out in subsequent operations of the data storage device.
Thus, one of the requirements of hard disk drive design is the provision of a degauss feature to demagnetize or degauss the write head after a write process has been performed. A typical method that has been used is to turn on an oscillator that provides a signal that becomes the input to the write driver for a number of oscillator cycles. After the oscillator is turned on, the amplitude of the write current is caused to decay from an initial write current setting to very close to zero, after which the oscillator and write driver are turned off.
Another requirement of the degauss feature is that the final decay value of the write current should be close to zero, independent from the initial write current setting. This means that for different initial write currents, at the end of the degauss time the final write current needs to always be the same, close to zero.
In the past, one degaussing circuit that has been used has a write current control circuit that has a digital-to-analog converter (DAC) which has a number of MOS transistors connected in parallel that may be selectively enabled to carry the output write current. The magnitude of the write current is dependent on the number of DAC transistors that are enabled and the control voltage that is applied to the gates of the enabled transistors.
The control voltage on the gates of the enabled DAC transistors is established by an input bias circuit, which, in the normal write mode, has a current source that provides a current through an input MOS transistor. The voltage developed across the input MOS transistor directly controls the control voltage on the gates of the enabled DAC transistors, which, in turn, controls the magnitude of the write current. In the normal write mode, the gate of the input MOS transistor is charged to a normal operating voltage, which establishes the amplitude of the write current.
When a degauss enable signal occurs after a data write operation, the current source to the input MOS transistor is disconnected, and a resistor is connected between the gate of the input MOS transistor and ground to discharge the normal operating voltage on the gate. As the voltage on the gate discharges, the control voltage on the gates of the DAC transistors decreases, thereby decreasing the write current.
There are several problems with this method. The voltage on the input MOS transistor is process dependent. For example, weak and strong MOS devices will produce different control voltages. Different control voltages results in different times that it takes for the normal operating voltage on the gate of the input MOS transistor to discharge down to the threshold value of the DAC transistors, which is the point where the write current equals zero.
Additionally, depending on the number of DAC transistors that are enabled, the total capacitance at the gate of the input MOS transistor may greatly vary. This results in different time decay constants, depending on the number of enabled DAC MOS transistors.
Finally, the decay of the output current is uncontrollably nonlinear.
Thus, what is needed is a circuit and method that provides a degaussing current that is process independent, is independent of the number of enabled DAC transistors, can be controlled to follow virtually any decay pattern, and can be controlled to decay to essentially zero from any initial starting current value.